The present invention is directed to an apparatus for locating magnetizable material in a structure and includes a scanning head containing a magnet for producing a magnetic field in the structure with the magnet having a first pole and a second pole. At least two magnetic field sensors are secured in the region of the magnetic poles for measuring the magnetic field disturbed by the magnetizable material. An evaluation unit is connected to the sensors for locating the magnetizable material by signals received from the sensors.
Such apparatus is known from the Andreas Schaab, et al. publication "Die zerstorungsfreie Prufung der Betondeckung der Bewehrung" (the non-destructive testing of concrete covering reinforcement) in Beton- und Stahlbetonbau (1989), pages 275-279 and 324-327. This known apparatus includes a scanning head or pickup with a permanent magnet for inducing a magnetic field in the structure and two magnetic field sensors fastened to the permanent magnet in the region of its poles for measuring the magnetic field disturbed by the magnetizable material. The permanent magnet is formed as a bar magnet and has one of the magnetic field sensors at each of the two pole regions at opposite ends of the magnet. An evaluation unit locates the magnetizable material by means of the measured signals received from the magnetic field sensors. The permanent magnet is placed with one of its end faces on the structure, such as a concrete member, and its magnetic flux changes as it approaches a reinforcing rod within the concrete member. To be more precise, on approach of the scanning pickup to a reinforcing rod located within the concrete member, the magnetic flux in the magnetic field located between the bar magnet and the structural member increases. This magnetic flux is checked by the sensors and is evaluated by a measurement amplifier and indicates the concrete covering the reinforcing rod.